For example, consider a
Matérn kernel of order \(s\) , which has the form
\(k(x,y)=C_2B_s({x-y})\exp (-C_1 {x-y})\) , where
\(C_1\) , \(C_2\)  are constants, and \(B_s(d)\)  is a modified
Bessel function of order \(s\) 
[1]}. This class of kernels
exhibits asymptotically polynomial decay of eigenvalues
[2]}.
